1. Field of the Invention
The present invention relates to a process for producing a copolymer of methyl methacrylate and styrene having excellent transparency, weatherability, heat stability, and solvent resistance in accordance with a solution process.
2. Description of the Related Arts
Copolymers of methyl methacrylate and styrene have excellent solvent resistance, dimensional stability, and moldability and low water absorption in addition to excellent transparency, stiffness, and weatherability and beautiful appearance which methacrylic resins generally exhibit. Therefore, these copolymers can be used as materials which have low densities, can be handled easily, and are economical and are used in various fields, such as signs, displays, covers for lights, and other building materials and electric parts. These copolymers are superior to methacrylic resins as optical materials because of higher refractive index and better dimensional stability and advantageously used in the field of optical sheets in recent years.
Heretofore, batch suspension polymerization processes and continuous bulk polymerization processes were generally used as the processes for producing copolymers of methyl methacrylate and styrene.
In the suspension polymerization, copolymers having different compositions are formed with progress of the copolymerization because the process is conducted batchwise, and homogeneity of the produced polymer is lost. This causes problems in quality, for example, in that the product becomes a mixture of copolymers having different diffraction indices and shows inferior transparency with turbidity.
To solve the above problem, a process in which a monomer mixture comprising styrene and methyl methacrylate in a specific range of amounts is polymerized by adding a specific polymerization initiator and a specific chain transfer agent, is disclosed in Japanese Patent Application Laid-Open No. Showa 55(1980)-16015. However, the applicable composition of the copolymer is limited to a narrow range around the azeotropic composition. A process in which, after the conversion of methyl methacrylate reaches 80% or more, copolymerization of methyl methacrylate and styrene is conducted with removal of a portion of unreacted monomers to the outside of the reaction system, is disclosed in Japanese Patent Application Laid-Open No. Showa 57(1982)-153009. However, this process requires complicated procedures in the steps of recovery of the monomers and purification of the copolymer and is not practical.
Moreover, contamination of the product with materials used in the process, such as emulsifiers and suspension dispersants, cannot be prevented in the suspension polymerization, and the product is not suitable for applications in which excellent optical properties are required. The suspension process has another problem as an industrial process in that complicated production steps, such as filtration, washing, drying, and waste water treatment, are included.
As the process for improving the drawbacks of the suspension polymerization, continuous bulk polymerization and solution polymerization can be mentioned. These polymerization processes are attracting attention because high quality polymers exhibiting excellent optical properties can be obtained in a wide range of the copolymer composition.
To produce copolymers of methyl methacrylate and styrene in accordance with the continuous bulk polymerization, a process in which a monomer mixture of styrene and methyl methacrylate having a specific composition and containing a radical generating catalyst is supplied continuously to a loop type reactor with coils and pipes to polymerize the monomers in a homogeneous phase, and a produced polymer is separated after the polymerization product is taken out, is reported in Japanese Patent Publication Showa 44(1969)-23506.
However, this process has a drawback in that continuous operation of the polymerization with stable control at a high conversion is very difficult because of an automatic accelerating effect by the so-called "gel-effect" and an increase in the viscosity of the polymerization product, and the conversion can be kept at most at about 60%. Therefore, a large amount of energy is necessary in the step of recovery and recycling of unreacted monomers and removal of volatile matters for concentration of the polymer. The polymer tends to be colored or degenerated by heat history during these treatments.
On the other hand, it is known that, in the solution polymerization, the gel effect described above can be suppressed by a decrease in viscosity of the polymerization product, and the conversion is increased. However, the solution polymerization has problems in that, unless the amount of the solvent is kept to the minimum, the total amount of volatile matters is not much different from that in the bulk polymerization even when the residual amount of monomers in a polymerization product is decreased, and the energy required for removing volatile matters is not much different from that in the bulk polymerization, either, and that processes for recovery and recycling of the monomers and the solvent become more complicated.
In Japanese Patent Publication No. Showa 44(1969)-23506 described above, a solution polymerization process using a small amount of an inert liquid diluent is proposed. However, no specific examples are shown. As for the continuous solution polymerization process, a process in which a general use solvent, such as an alkylbenzene, an aliphatic hydrocarbon, an ester, or a ketone, and a stabilizer are added to a monomer mixture containing styrene and methyl methacrylate, and copolymerization is conducted continuously in a plurality of reactors, is reported in Japanese Patent Application Laid-Open No. Showa 57(1982)-135814.
However, the solvents described in the above reference show poor volatility or have unnecessarily high affinity with the polymer. The polymerization product must be treated generally at a high temperature exceeding 220.degree. C. to remove volatile matters when copolymers of methyl methacrylate and styrene having a content of residual volatile matters of 0.3% or less should be obtained as required for practical application. When the polymerization product containing a large amount of unreacted monomers is pre-heated to a high temperature exceeding 220.degree. C., a problem arises in that a large amount of low molecular weight polymers are formed to cause a wider molecular weight distribution, and fluidity under heating and transparency of the obtained polymer becomes inferior. Moreover, when a polymerization product which contains a solvent having poor volatility or a solvent having unnecessarily high affinity with the polymer is introduced directly into an extruder after pre-heating, a problem arise in that tubing of vents in the area of the inlet are choked with the resin composition, i.e., the so-called vent-up takes place, and difficulty in the operation and contamination of the product are caused.